This invention relates to the surface treatment, in particular coating, of rotors having a profile which progresses helically along the rotor.
Down-hole drilling motors have a multi-lobed rotor which is surrounded by an elastomer stator with xe2x80x9cnegativexe2x80x9d lobes which mate with the lobes of the rotor. The lobed profile scrolls down the length of the rotor (which can be up to 6 meters in length) and the spiral path of the lobes often wraps around the rotor length more than one full turn. The stator has an extra lobe, which allows drilling muds to be pumped down the motor, and the force of these fluids imparts a rotary motion to the rotor, which provides the driving force for the drill bits attached to the end of the motor.
In the past, the rotors have been chromium plated to protect them from corrosion and to provide a surface compatible with that of the elastomer stator. However, wells are now being drilled in more difficult geological structures and this requires drilling muds which are more corrosive because of the content of various salts (e.g. sodium chloride) which can be as high as 300,000 ppm. Hard chromium plating always contains cracks and the corrosive drilling muds can penetrate these cracks and initiate corrosion between the chromium plate and the substrate material. In a very short space of time (sometimes as little as 20 hours) the corrosion products cause the chromium plate to separate from the substrate material and these separate pieces of chromium together with the corrosion products themselves attack and destroy the profile of the elastomer stator, which in turn reduces the drilling performance of the motor to unacceptable levels.
It has now been demonstrated that a composite WC/ceramic coating such as described in GB-A-2 269 392 can out-last chromium plating to such an extent that the new coating is being considered as a replacement for chromium plating of rotors. However, there are many thousands of rotors in the industry""s xe2x80x9cfleetsxe2x80x9d and all of these rotors have been machined to specific sizes to accept the normal chromium plating thicknesses. The size tolerances between rotor and stator are obviously of major importance in defining motor performance. Because the chromium plate is applied electrolytically, the lobe peaks generate a higher current density in the plating baths than the valleys and, consequently, a thicker coating is deposited on the peaks (450-500 xcexcm) than is deposited in the valleys (75-100 xcexcm) between the peaks. In contrast a WC/ceramic composite coating is deposited by the combination of a high velocity oxygen-liquid fuel (HVOF) technique and a thermochemical deposition technique, but coating thickness is largely dictated by the HVOF technique.
The HVOF technique is a particular form of flame spraying technique. A cylindrical component to be coated is revolved at a precise speed whilst the deposition xe2x80x9cspotxe2x80x9d generated by the coating torch or gun is traversed along the length of and on the centre-line of the component at a speed which is matched to the speed of rotation so that the spot follows a tight helical path. A coating of uniform longitudinal thickness is achieved when the pitch of the helical deposition path is less than the diameter of be deposition spot. The cross-sectional thickness of an HVOF coating deposited in this matter on a rotor is effectively dictated by the major and minor diameters of the rotor profile. Thus, if the major diameter (lobe peak) is twice that of the minor diameter (lobe valley), then the coating thickness in the valleys will be twice that of the peaks. Depending upon the number of lobes on a rotor (and thus the slope of the valley sides) the thickness in the valleys can be further increased by a funnelling or concentrating effect on the coating deposition spot.
Thus the natural coating thickness profile deposited by the conventional HVOF technique is completely opposite to that of chromium plating techniques and they are different to such an extent that the desired xe2x80x9cfitxe2x80x9d between rotors and standard stators can only be achieved by machining a new rotor to a specific size to accommodate the natural coating profile of the conventional HVOF coating. However, this would mean that the many thousands of rotors in the existing fleets could not be coated by the WC/ceramic composite coating technique and many of them would have to be scrapped because of the severity of the corrosion problems. Because of the slope variations created by the major and minor diameters of the cross-sectional profile of the rotors, it is difficult to ensure that the angle between the coating deposition plane and the coating stream is maintained at the optimum 90xc2x0 when coating on the longitudinal centre-line of the rotor. Thus coating quality and bond strength cannot be optimised uniformly around the rotor.
The present invention provides a method of treating the surface of a rotor having a profile which progresses helically along the rotor, comprising providing a treatment jet or beam having an axis intersecting the surface of the rotor at a point, and traversing the point along the rotor while keeping the point at the same position on the profile.
The invention also provides apparatus for performing the method.